For many years, the art has recognized the desirability of liquid formulations of biologically active compounds such as pesticides and growth regulants. However, many of the interesting biologically active compounds suitable for these end uses are solids at room temperature and are too expensive and too powerful in effect to be used in most applications without dilution with inert ingredients. Two general approaches to this problem have been developed. In one, the pesticide manufacturer or formulator dilutes the active ingredients with other solid materials in such a manner that the total formulation can then be readily dispersed into a liquid medium, usually water, by the ultimate end use or applicator. Such formulations include the wettable powders and water dispersible granules which are also known as dry flowable. The other approach has been to disperse or dissolve the active ingredient in a liquid medium in such a manner that it can either be used directly in such applications as ultra low volume spraying or can be diluted by the end user or applicator in liquid medium such as water or ecologically tolerable oils.
These liquid formulations pose the problem of stability both during the time it takes to distribute the product to the ultimate end user or applicator and also during the period between application seasons when the end user does not expend all of the purchased material during a given application season. Both solutions and dispersions can display instability but the solutions are usually more stable. They can become unstable if they are exposed to conditions such as cold temperatures which cause some of the dissolved active ingredient to precipitate. These precipitated crystals may not readily redissolve under conditions conveniently obtainable in the field. However, the major problem with the solution approach to formulations, is finding solvents for the biologically active compounds which are acceptable from both a cost and ecological viewpoint. Many organic solvents have been identified as being too hazardous to either human, animal or plant life to be the carriers or part of the carriers used to apply biologically active compounds, particularly in the fields of agriculture and forestry management where the application is over large open areas.
These concerns have been met by dispersing active ingredients in relatively inexpensive and ecologically acceptable liquid medium but this approach has problems of its own with regard to stability, viscosity and dilutability. The tendency of the solid particles of a dispersion to settle out or separate from the liquid medium in which they are dispersed can be inhibited by reducing the particle size of the dispersed material and by increasing the viscosity of the dispersion or suspension. In fact, in may cases just reducing the particle size of the dispersed material will increase the viscosity of the dispersion. However, a dispersion with too high a viscosity is not commercially acceptable because of the problems it poses in handling the material. "Liquid" formulations which can not be readily poured or pumped are decidedly unattractive to both manufacturers and end users. On the other hand, the less viscous a dispersion is, the greater tendency there is for the dispersed particles to separate from the dispersing medium.
A related concern is how "redispersible" a separated dispersion is. In general, the larger the particle size of the dispersed material, the more difficult it will be to redisperse it should any settling occur. This phenomenon, known as "caking", is generally less of a problem with the finer particle size dispersions although this may not be true with extremely fine particles in the range of about 1 micron and less.
Another concern with dispersions is that the dispersing medium not dissolve any significant portion of the dispersed material. Because there is, by definition, in a dispersion more dispersed material present than will dissolve in the dispersing medium, it is almost certain that any lowering of the temperature of the dispersion below the highest temperature to which it has been exposed, will result in some precipitation of previously dissolved material. Any significant precipitation will result in the formation of precipitated crystals that may cause significant difficulties in the further handling of the dispersion. For instance, if the dispersion is used in a spray application such crystals may cause the plugging of the spray or pumping apparatus used. However, while many liquids which are attractive as dispersing medium are poor solvents for many attractive active ingredients they are not complete non-solvents. The solution has generally been to manufacture dispersions at temperatures as close to ambient as possible. At such temperatures (around 20.degree. C.) the solubility is usually so low that any precipitation which does occur upon exposing the dispersion to lower temperatures is not significant enough to be of concern.
This temperature constraint does pose some inconveniences for the manufacturer of dispersions. He would like to manufacture the dispersion at as high a concentration as possible and then dilute it for ultimate shipment. Such a procedure maximizes the efficiency with which the capital equipment, particularly the sand mill or other grinding equipment committed to such production, are utilized. However, as the concentration of the dispersion being produced increases so does its viscosity until the point is reached at which the dispersion can no longer be properly processed in the grinding equipment. For instance, if the viscosity of a dispersion being processed in a sand mill becomes too high the mill will be unable to provide the desired particle size reduction. Dispersions with too large a particle size are expected to display inadequate stability and caking. Raising the temperature at which the milling operation is conducted would lower the viscosity and enable the milling operation to be conducted at higher concentrations of dispersed material but it would also pose the significant danger of dissolving significant amounts of the dispersed material. Thus, elevated temperature milling has been avoided in those cases in which the dispersed material has any significant elevated temperature solubility in the dispersing medium.
Liquid pesticide formulations must generally be dilutable with liquid medium. It is economically unattractive to transport liquid pesticide formulations which have a sufficiently low active ingredient content to be directly applicable by most end users because this would involve freight costs for transporting a large amount of inert materials. Instead such formulations are typically produced with concentrations of active ingredients convenient for manufacturing, handling and storage. However, this means that the liquid formulation must be readily dilutable with materials available to the end user. Typical materials include water and oil. But dilution increases the problem of dispersion stability because it lowers the viscosity of the dispersion. In fact, in the case of the oil dilution some seprataion of the diluent oil from the pesticide concentrate has come to be accepted so long as the uniform dispersion may be readily reestablished. Thus, the problem of "caking" or the formation of nonredispersible sediments is of particular concern with regard to such dilute final formulations.
A convenient test for the degree of redispersibility has been developed which has a reasonable correlation with actual field experience. The dilute dispersion to be evaluated is placed into a number of sealed containers which are stored for various periods of time at various temperatures. Each container is evaluated by inverting it 180.degree. and holding it in this position until the heavier material now at the top ceases to flow downward and then returning the container to its original position. The redispersibility of the dilute dispersion is evaluated by the number of such inversions which are necessary to produce a uniform dispersion after some settling has occurred.
Material which reforms a uniform mixture in 8 or less inversions has been found to be readily remixable in commercial spray equipment. For instance, such material poses no problems when used in 10,000 gallon tanks with recirculating pumps utilized in applying insecticide in forestry management. The agitation supplied by the recirculation pump and the design of the feed pipes is more than adequate to remix such materials to a uniform consistency after some separation has occurred.
An attractive oil based liquid pesticide concentrate with aminocarb as the active ingredient has been marketed for several years. It involved about 21 wt % of active ingredient, between 3 and 4 wt % of a hydrated silica having a surface area in excess of 100 square meters per gram, and about 0.5 wt % of a soy bean oil derived lecithin dispersed in an aliphatic petroleum fraction having a Saybolt Universal viscosity at 100.degree. F. of less than about 100 seconds. The dispersed aminocarb had an average particle size less than about 10 microns. However, this product was produced by milling at temperatures less than 40.degree. C. and as a result it was unable to provide the optimum combination of low viscosity and easy redispersibility especially after dilution in diesel oil.